One of the most conspicuous features of many eukaryotic<br >cells is the high degree of development of their cyto-<br >membrane system. In some cells， the total surface area of<br >this system may exceed 0.1 mm2， which may not seem<br >like much until you adjust it to our millionfold magnifica-<br >tion. Then you find that it means dividing up the space of<br >a large auditorium with more than one million square feet<br >of partitions. Moving through such an auditorium may be<br >a problem， as indeed it is in the ER-rich areas of the cell，<br >where flattened cisternae may be packed so close together<br >as to leave only very narrow passages between them. For-<br >tunately， the partitions are flexible and can be forced apart<br >fairly easily， as shown by the occasional mitochondrion<br >or other cytoplasmic granule<br >cisternae.<br >found nestling between ER<br >Membranes as Organelles<br > During the first part of our tour， we were given<br > plenty of proof that these membranes are not mere<br > partitions. But relatively little of what they do， and<br >especially of how they do it， could be discerned on their<br >trans face. And so our descriptions had to remain largely<br >phenomenological. Now that we can inspect the cis face<br >of the membranes， we get a much clearer view of the<br >number and variety of active systems that are compressed<br >in these tenuous films. Even more important， we can try<br >to find out how they operate， as their works are largely<br >exposed on the side we are facing， which is where they are<br >supplied with substrates， receive energy， interact with<br >